An existing plastic pump is shown in FIG. 7, where the motor is not shown. The water inlet pipe 1 of the pump casing includes four sections in serial: starting section 2, first middle section 3, second middle section 4, and end section 5. The starting section 2 is tilted with respect to the axis of the motor. The first middle section 3 is substantially trapezoidal and substantially perpendicular to the axis of the motor, with the opening of smaller diameter connected to the starting section 2. The second middle section 4 bends substantially 90 degrees. The end section 5 is connected to the second middle section 4 and is substantially parallel with the axis of the motor. Water enters the starting section 2, passes through the first and second middle sections 3 and 4, and flows into an impeller 6 via the end section 5.
As the opening of the first middle section 3 having greater diameter is spaced from the starting section 2, a die for producing the first middle section 3 cannot be extracted from the staring section 2. That is to say, the starting section 2 and first middle section 3 cannot be made integrally in a single injection process. Similarly, as the second middle end 5 bends through a large angle, the first middle section 3 and the second middle section 4 cannot be made integrally in a single injection process. Thus, the whole pump casing cannot be made as a single piece plastic injection molding.